The invention relates to a method of controlling the mold filling process in a plastics injection molding apparatus, which has a filling phase with a controlled speed for the injection piston and a post-pressure on back-presure phase with a controlled hydraulic pressure applied to the injection piston, wherein the injection work which is performed between two positions of the injection piston is determined and the change-over between the filling phase and the post-pressure phase is effected independence on the detected value of the injection work.
It is known for example from PLASTverarbeiter 34, volume 1983, No. 10, pages 1105-1108, that monitoring the relationship between hydraulic pressure and flow speed makes it possible in particular to compare the viscosities of the molding materials. The same work also already gave a fairly complicated process for determining the value in respect of the injection work, that corresponds to complete filling of the mold, the attainment of that value then supplying the signal for the step of changing over to the post-pressure phase. Correction of the post-pressure independence on the injection work which is detected in the same cycle is indicated therein as a possibility.
It has also already been proposed in DE-A-26 05 037 that the level of the post-pressure may be altered in order to avoid fluctuations in the desired quality of the molding, which would have been expected without that alteration on the basis of measurements made in the same cycle. In that process, just as for example in accordance with GB-A-1 459 673, measurements are made in particular in respect of the internal pressure in the mold and the time at which it reaches a maximum, which indicates complete filling of the mold.
Continuously monitoring the internal pressure of the mold has been found to be difficult, particularly in regard to the production of moldings of a complicated configuration. At any event it represents an operation which increases the moulding costs.
The present invention is therefore based on the problem of providing for information about the molding quality to be expected, on the basis of measurements which are made in the production of a molding at the injection molding apparatus itself, and thereby supplying a parameter which makes it possible to prevent an undesired change in quality by influencing the manufacturing process.
In solving the above-indicated problem, it was first found that influencing the level of the post or back-pressure at a time when the internal pressure of the mold has already exceeded its maximum and the mold is therefore completely filled, has only a slight influence on the final result. It was surprisingly found however that, in order to avoid fluctuations in quality, in particular in the mass of the moldings due to the material not being of a uniform nature, the change-over point between the filling phase and the post-pressure phase is displaced by a predetermined value in dependence on the injection work which is detected during the filling phase. In accordance with the invention therefore the effect of fluctations in material may be compensated by virtue of the fact that the change-over point between the filling phase and the post-pressure or back-pressure phase is not only approximately equated to the moment of complete filling of the mold, whereas the level of the post pressure is selected independently of the fluctuating quality of material. The underlying reason for that mode of operation is based on the following consideration: as is already known, for example the mass of the moulding fluctuates independence of the viscosity of the material used, in respect of which the injection work WE between two predetermined positions of the piston forms a measurement. Hitherto that piece of knowledge has only been used to treat automatically as waste moldings in which the deviation, as detected in the injection cycle, in respect of the injection work from the normal value, leads to the expectation of an excessive deviation in the mass of the molding from the normal value.
The novel and surprising factor was to find that, by virtue of influencing a single parameter, namely the change-over point between the filling phase and the post-pressure phase, it is possible to compensate for the influence of all fluctuations in the quality of the material, which are involved in the value in respect of the injection work. In the first instance attention would be directed in this case to a change in temperature in order to adjust the viscosity to a normal value, but changes in temperature can only be attained with a considerable delay and in addition they have a reaction effect on the entire system. A change in the speed of the piston in the filling phase, which in itself would be possible, would at most be possible in the cycle following the measurements in respect of the injection work. Correction of the post- or back-pressure, which is proposed in the publication referred to in the opening part of this specification, becomes increasingly ineffective with increasing progress in the post-pressure phase, that is to say, as the sealing point is approached. An alternative expedient is afforded by virtue of departing, in the manner proposed according to the invention, from the requirement which in itself is soundly based from the theoretical point of view (see PLASTverarbeiter 34th volume 1983, No. 5, page 430), of causing the time of changing over to the post-pressure phase, that is to say, from the condition of flow which is governed by the machine to the flow which is governed by the tool, to coincide precisely with that moment at which the mold is just filled.
The relationship between the extent to which the change-over point is shifted and the injection work which depends on the material is to be established in accordance with the invention in such a way that, for various materials which differ slightly in regard to their properties, the change-over point is shifted until the desired quality of molding and in particular the desired mass or the desired dimensions, are attained. If the relevant value for the injection work is plotted in a graph, interpolation gives that change-over point which is to be selected for a material which is still unknown and in respect of which the injection work has been ascertained in the first part of the injection cycle.